The present invention relates to rotary fluid pressure devices, and more particularly, to such devices which have the capability of operating in a free-wheeling mode.
Although the present invention may be utilized in connection with various types of rotary fluid pressure devices, and to such devices having various types of fluid displacement mechanisms, it is especially adapted for use with low-speed, high-torque orbiting gerotor motors, and will be described in connection therewith.
Low-speed, high-torque gerotor motors have been in commercial use for many years and are especially suited for applications such as vehicle wheel drives, winch drives, and providing rotary torque to various other vehicle implements. Motors of this type have been commercially successful, partially because the gerotor gear set is uniquely suited to provide the desired low-speed, high-torque output in a device which is both compact and relatively inexpensive.
In many of the applications for gerotor motors of the type noted above, it has been found desirable to be able occasionally to operate the motor in some mode other than its normal, operating mode. For example, if the motor is being used to provide torque to the drive wheels of the vehicle, it would be quite useful to be able to operate the motor in a free-wheeling mode when the vehicle is being towed. Typically, when a vehicle which is normally propelled by a gerotor motor is being towed, the output shaft of the motor is being driven by the wheels and, in turn, the gerotor gear set is being driven by the output shaft. In order to avoid the problem of the gerotor motor acting as a dynamic brake, an open center directional control valve is normally used in such applications, thus permitting the fluid to recirculate through the open-center valve during towing.
The fact that the gerotor gear set is being driven during towing has a number of disadvantages. A vehicle which is normally propelled by a gerotor motor is typically towed at a speed much greater than its normal operating speed. Thus, the speed of movement of the gerotor elements and the associated shafts and splines is greater than during normal operation, which can result in damage due to excessive heating of the gerotor elements, splines, etc. Also, this higher speed movement of these elements results in the generation of substantial heat in the fluid which can damage various other parts of the overall hydraulic system. It should be noted that during towing of such a vehicle, the vehicle engine is off, with the result that there is no fan operating to cool the fluid in the vehicle hydraulic system. Finally, because the gerotor motor acts like a pump during such towing operation, a greater amount of horsepower is required to tow such a vehicle.
An attempt by the prior art to provide a motor capable of operating in a free-wheeling mode is illustrated by U.S. Pat. No. 4,435,130. Although the device disclosed therein is described as having a free-wheeling mode, the actual mode of operation is to establish a short-circuit flow path from the inlet port to the outlet port, across the commutating valve. Thus, the commutating valve operates somewhat like the open-center directional control valve referred to previously. Because the motor output shaft is still connected to the rotating element of the gerotor, the volume chambers within the gerotor are still expanding and contracting in response to rotation of the output shaft. Such a motor is not in a true free-wheeling mode, and still suffers from each of the disadvantages described above, at least to some extent.
An arrangement which achieves a true free-wheeling mode of operation is illustrated and described in co-pending application U.S. Ser. No. 697,596, filed in the name of M. L. Bernstrom and S. J. Zumbusch for a "hydraulic motor having free-wheeling and locking modes of operation", assigned to the assignee of the present invention. The arrangement covered by the co-pending application makes it possible to physically disconnect the motor output shaft from the gerotor gear set to achieve free-wheeling.